CO2 removal from a gas stream may be obtained using chemical and physical absorption processes. Chemical absorption of CO2 may be performed with amine based processes and alkaline salt-based processes. In such processes, the absorbing medium reacts with the absorbed CO2. Amines may be primary, secondary, and tertiary. These groups differ in their reaction rate, absorption capacity, corrosion, degradation, etc. In alkaline salt-based processes, the most popular absorption solutions have been sodium and potassium carbonate. As compared to amines, alkaline salt solutions have lower reaction rates with CO2.
Alkanolamines in aqueous solution are another class of absorbent liquid for carbon dioxide removal from gaseous mixtures. Alkanolamines are classified as primary, secondary, or tertiary depending on the number of non-hydrogen substituents bonded to the nitrogen atom of the amino group. Monoethanolamine (HOCH2 CH2NH2) is an example of a well-know primary alkanolamine. Widely used secondary alkonalamine include diethanolamine ((HOCH2CH2)2NH). Triethanolamine ((HOCH2CH2)3N) and methyldiethanolamine ((HOCH2CH2)2NCH3) are examples of tertiary alkanolamines which have been used to absorb carbon dioxide from industrial gas mixtures. Molecular structures of sterically hindered amines are generally similar to those of amines, except sterically hindered amines have an amino group attached to a bulky alkyl group. For example, 2-amino-2-methyl-1-propanol (NH2—C(CH3)2CH2OH).
With primary and secondary alkanolamines (Pinola et al. Simulation of pilot plant and industrial CO2-MEA absorbers, Gas Separation & Purification, 7(1), 1993; Barth et al., Kinetics and mechanisms of the reactions of carbon dioxide with alkanolamines; A discussion concerning the cases of MDEA and DEA, Chemical Engineering Science, 39(12), pp.1753-1757, 1984) the nitrogen reacts rapidly and directly with carbon dioxide to bring the carbon dioxide into solution according to the following reaction sequence:2 RNH2+CO2RNHCOO−+RNH3+  (1)
where R is an alkanol group. This reaction is the cornerstone of the present invention, as it is the one accelerated by carbonic anhydrase. The carbamate reaction product (RNHCOO−) must be hydrolysed to bicarbonate (HCO3−) according to the following reaction:RNHCOO−+H2ORNH2+HCO331   (2)
In forming a carbamate, primary and secondary alkanolamine undergo a fast direct reaction with carbon dioxide which makes the rate of carbon dioxide absorption rapid. In the case of primary and secondary alkanolamines, formation of carbamate (reaction 1) is the main reaction while hydrolysis of carbamate (reaction 2) hardly takes place. This is due to stability of the carbamate compound, which is caused by unrestricted rotation of the aliphatic carbon atom around the aminocarbamate group. According to U.S. Pat. No. 4,814,104 the overall reaction for the alkanolamines is written as:2 RNH2+CO2RNHCOO−+RNH3+  (3)
For the sterically hindered amines both reactions 1 and 2 play major roles on the CO2 absorption process. In contrast with the alkanolamines, the rotation of the bulky alkyl group around the aminocarbamate group is restricted in sterically hindered amines. This results in considerably low stability of the carbamate compound. The carbamate compound is thus likely to react with water and forms free amine and bicarbonate ions (reaction 2). Due to the occurrence of reaction 2, only 1 mol of the sterically hindered amine instead of 2 mol of alkanolamine is required to react with 1 mol of CO2. The overall reaction for sterically hindered amines can be written as (Veawab et al., “ Influence of process parameters on corrosion behaviour in a sterically hindered amine-CO2 system”, Ind.Eng.Chem.Res., V 38, No. 1; 310-315; 1999; Park et al., Effect of steric Hindrance on carbon Dioxide Absorption into New Amine Solutions: Thermodynamic and Spectroscopic Verification and NMR Analysis, Environ. Science Technol. 37, pp.1670-1675, 2003; Xu, Kinetics of the reaction of carbon dioxide with 2-amino-2-methyl-1-propanol solutions, Chemical Engineering Science, 51(6), pp.841-850, 1996):RNH2+CO2+H2ORNH3+HCO3−  (4)
Unlike primary and secondary alkanolamines, tertiary alkanolamines cannot react directly with carbon dioxide, because their amine reaction site is fully substituted with substituent groups. Instead, carbon dioxide is absorbed into solution by the following slow reaction with water to form bicarbonate (U.S. Pat. No. 4,814,104; Ko, J. J. et al., Kinetics of absorption of carbon dioxide into solutions of N-methyldiethanolamine+water, Chemical Engineering Science, 55, pp.4139-4147, 2000; Crooks, J. E. et al., Kinetics of the reaction between carbon dioxide and tertiary amines, Journal of Organic Chemistry, 55(4),1372-1374, 1990; Rinker, E. B. et al., Kinetics and modelling of carbon dioxide absorption into aqueous solutions of N-methyldiethanolamine, Chemical Engineering Science, 50(5), pp.755-768, 1995):R3N+CO2+H2OHCO3−+R3NH+  (5)
Physical absorption enables CO2 to be physically absorbed in a solvent according to Henry's law. Such absorption is temperature and pressure dependent. It is usually used at low temperature and high pressures. Typical solvents are dimethylether of polyethylene glycol and cold methanol.
In recent years, a lot of effort has been put to develop new absorption solutions with enhanced CO2 absorption performance. The use of sterically hindered amines, including aminoethers, aminoalcohols, 2-substituted piperidine alcohols and piperazine derivatives, in solution to remove carbon dioxide from acidic gases by scrubbing process was the object of a patent in the late 1970 (U.S. Pat. No. 4,112,052). Yoshida et al. (U.S. Pat. No. 5,603,908) also used hindered amines to remove CO2 from combustion gases, but mainly focused on reducing the energy consumption from the amines regeneration. Fujii et al. (U.S. Pat. No. 6,274,108) used MEA in a process to absorb CO2 from combustion exhaust gases, but were more concerned about the plant design, more specifically storage of the amines and replenishing system. Instead of using amines, Suzuki et al. used various formulations of amino-amides to remove carbon dioxide from gases and absorbent (U.S. Pat. No. 6,051,161).
In literature, some have reported new formulations of absorption solutions for chemical and physical processes. Reports exist about the reduction of corrosion of carbon steel with the use of certain amine compounds (U.S. Pat. No. 6,689,332). These new formulations may imply mixtures of amines (chemical solvent). For instance, patent U.S. Pat. No. 5,246,619 discloses a way of removing acid gases with a mixture of solvents comprising methyldiethanolamine and methylmonoethanolamine. Mixtures of dialkyl ethers of polyethylene glycol (physical solvent) (U.S. Pat. No. 6,203,599), and mixtures of chemical and physical solvents are reported. GB 1102943, for instance, reports a way of removing CO2 by using a solution of an alkanolamine in a dialkyl ether of a polyalkylene glycol, while U.S. Pat. No. 6,602,443 reduces CO2 concentration from gas by adding tetraethylene glycol dimethyl ether in combination with other alkyl ethers of alkylene glycols. Although U.S. Pat. No. 6,071,484 describes ways to remove acid gas with independent ultra-lean amines, mention is also made that a mixture of amines and physical absorbents can also be used with similar results.
In order to increase the rate of CO2 absorption, especially for aqueous tertiary alkanolamine solutions, promoters have been added to the solutions. Promoters such as piperazine, N,N-diethyl hydroxylamine or aminoethylethanolamine (AEE), is added to an absorption solution (chemical or physical solvent). Yoshida et al. (U.S. Pat. No. 6,036,931) used various aminoalkylols in combination with either piperidine, piperazine, morpholine, glycine, 2-methylaminoethanol, 2-piperidineethanol or 2-ethylaminoethanol. EP 0879631 discloses that a specific piperazine derivative for liquid absorbent is remarkably effective for the removal of CO2 from combustion gases. Peytavy et al. (U.S. Pat. No. 6,290,754) used methyldiethanolamine with an activator of the general formula H2N—CnHn—NH—CH2—CH2OH, where n represents an integer ranging from 1 to 4. U.S. Pat. No. 6,582,498 describes a wire system to reduce CO2 from gases where absorbent amine solutions and the presence of an activator are strongly suggested. U.S. Pat. No. 4,336,233 relates to a process for removing CO2 from gases by washing the gases with absorbents containing piperazine as an accelerator. Nieh (U.S. Pat. No. 4,696,803) relied on aqueous solution of N-methyldiethanolamine and N,N-diethyl hydroxylamine counter currently contacted with gases to remove CO2 or other acid gases. Kubek et al (U.S. Pat. No. 4,814,104) found that the absorption of carbon dioxide from gas mixtures with aqueous absorbent solutions of tertiary alkanolamines is improved by incorporating at least one alkyleneamine promoter in the solution.
Other ways of enhancing CO2 absorption involve ionic liquids, more specifically a liquid comprising a cation and an anion having a carboxylate function (US 2005/0129598). Bmim-acetate and hmim-acetate are cited as examples.
Mention of enzyme utilization for gas extraction can also be found in the literature (U.S. Pat. Nos. 6,143,556, 4,761,209, 4,602,987, 3,910,780). Bonaventura et al. (U.S. Pat. No. 4,761,209) used carbonic anhydrase immobilized in a porous gel to remove CO2 in an underwater rebreathing apparatus. Carbonic anhydrase can also be used to impregnate membranes used to facilitate CO2 transfer into water for similar purposes (U.S. Pat. Nos. 4,602,987, 3,910,780). Efforts were made to ensure that the active site of the enzymes fixed on the membranes were in direct contact with the gas phase substrate to increase the activity of the enzymes (U.S. Pat. No. 6,143,556). This patent is the direct continuation of patent U.S. Pat. No. 6,524,843, which claimed a way to remove CO2 from gases with an enzyme, the carbonic anhydrase. This new patent aims at improving the CO2 absorption of the previous patent through the additional use of solvents, increasing the performance of the bioreactor.
CO2 transformation may be catalyzed by a biocatalyst. The biocatalyst is preferably the enzyme carbonic anhydrase. CO2 transformation reaction is the following:CO2+H2OHCO331 +H+  (6)
Under optimum conditions, the turnover rate of this reaction may reach 1×106 molecules/second (Khalifah, R and Silverman D. N., Carbonic anhydrase kinetics and molecular function, The Carbonic Anhydrase, Plenum Press, New York, pp.49-64, 1991).